1. Field of the Invention
The present invention relates generally to the field of electrical waveguide components used for driving a series push-pull traveling wave electrode Mach-Zehnder optical modulator. More specifically, the present invention discloses an electrical waveguide transmission device that receives at the input a differential pair of modulated electrical signals propagating along two separate signal conducts with grounded electrical return paths, and outputs the differential signal over a pair of output conductors that act as a return path for each other and provide a desired characteristic impedance matching that of the Mach-Zehnder modulator.
2. Background of the Invention
Mach-Zehnder optical modulators have been employed for many years in the field of optical communications to accept modulated data in electrical (e.g., radio frequency) format and transfer the data onto an optical carrier. In a Mach-Zehnder optical modulator, a beam splitter divides the laser light into two paths, at least one of which has a phase modulator in which the refractive index is a function of the strength of the local electric field. The beams are then recombined. Changing the electric field on the phase modulating path will then determine whether the two beams interfere constructively or destructively at the output, and thereby control the amplitude or intensity of the exiting light.
Some Mach-Zehnder optical modulators employ a series push-pull travelling wave electrode, as shown in FIG. 1, after Klein et al., “1.55 μm Mach-Zehnder Modulators on InP for optical 40/80 Gbit/s transmission networks”, OFC/NFOEC 2006, paper TuA2, and is described in further detail by R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators”, IEEE J. Quant. Elect., vol. 27(3), pp. 654-667, 1991. A travelling wave electrode (or TWE) consists of two or more transmission line conductors oriented substantially parallel to the optical paths, and a plurality of waveguide electrodes. Each waveguide electrode is connected to at least one of the transmission line conductors via a tap or bridge conductor. Each bridge conductor branches out of a transmission line conductor in a direction substantially perpendicular to the optical path. The transmission line conductors convey an RF signal in an RF path that is substantially parallel to the optical paths. Each pair of waveguide electrodes act as a pair of capacitors in series to each other and as a load to the main transmission line, and impart a phase change to the optical wave in the waveguide.
As shown in FIG. 1, a series push-pull travelling wave electrode Mach-Zehnder optical modulator typically includes: (1) an input optical waveguide 11 for receiving an input optical signal; (2) a splitting means 12 for splitting the optical signal into a first optical branch and a second optical branch; (3) first and second optical waveguides 14A, 14B conveying the light from the two branches of the optical signal, respectively; (4) two or more transmission line conductors 13A, 13B for receiving and conveying an input electrical signal; (5) a plurality of pairs of waveguide electrodes 17, 18 positioned adjacent to the first and second optical waveguides 14A and 14B, respectively, and electrically connected to the respective transmission line conductors 13A, 13B, so that the waveguide electrodes 17, 18 alter the phase of the optical signal in response to the input electrical signal; and (6) a combining means 16 recombines the beams at the output of the optical modulator 10. It should be noted that the optical and electrical signals propagate in substantially the same direction along the optical modulator 10.
The prior-art travelling wave modulator shown in FIG. 1 employs two transmission line conductors 13A and 13B, one of which carries the input electrical signal (S) and the other of which is connected to a reference or ground (G) potential. This is sometimes referred to an SG configuration. A conventional approach to driving this configuration, known as single-ended drive, is illustrated in FIG. 2. One output of an RF driver 20 having an output impedance 21 is connected to the S transmission line conductor of the optical modulator 10 via, e.g., the center conductor of a co-axial cable 25. The other output of the RF driver 20 is grounded and connected to the G transmission line conductor of the modulator 10 via, e.g., the outer conductor of a co-axial cable. A nominal terminal load 30 (e.g. 50 ohms) connects the distal ends of the travelling wave electrode. It should be noted that the modulation voltage across the S and G conductors of the travelling wave electrode is the difference between the signal voltage and ground. This configuration has the disadvantage of losing a large fraction of the electrical power supplied by the RF driver 20.
In contrast to Klein, other traveling wave Mach-Zehnder modulators use a configuration shown in FIG. 3, employing five transmission line conductors. This type of modulator is described, for example, by Tsuzuki et al., “40 Gbit n-i-n InP Mach-Zehnder Modulator with a π Voltage of 2.2 V”, Electronics Letters, vol. 39, no. 20, Oct. 2, 2003. The modulator consists of two independent signal transmission line conductors (S+ and S−), each with an adjacent ground transmission line conductor (G1 and G3, respectively), and with a ground transmission line conductor interposed between them (G2). Because of the interposed ground conductor G2, S+ and S− are electrically independent, and the current-return path of each is via G1/G2, and G2/G3, respectively. The nominal loads (e.g., two 100 ohm resistors in parallel) connect the distal ends of S+ with G1 and G2, and S− with G2 and G3.
A conventional approach to driving this configuration, known as differential drive, is illustrated in FIG. 4. Both outputs (S+and S−) of the RF driver 20 are connected through two waveguides (e.g., coaxial cables 25, 26) to the travelling wave electrodes, and the outer conductors of both coaxial cables 25, 26 are grounded. Both outputs of the RF driver have a characteristic output impedance 21, 22. In the case of differential drive modulation, the ground conductors of the two coaxial cables are connected to the ground transmission line conductors of the optical modulator 10. Nominal termination loads 30, 31 (e.g., 50 ohms for each of S+ and S−) are connected across the distal ends of the travelling wave electrodes. The outputs from the RF driver 20 are in anti-phase (i.e., S+ and S− are 180 degrees out of phase) and the modulation voltage across the travelling wave electrode is S+ minus S−. In this configuration both RF driver outputs are utilized, greatly improving the power efficiency compared to the single-ended drive condition.
A critical distinction between the optical modulator of Tsuzuki and the modulator of Klein is that Tsuzuki uses an independent traveling wave electrode for each of the first and second optical branches of the modulator. The signal lines of the device (S+ and S−) are electrically independent and have a ground return line interposed between them. In the case of a series push-pull Mach-Zehnder optical modulator as in FIGS. 1 and 2, the two transmission line conductors are coupled and act as a return path for each other. Therefore, Tsuzuki does not teach how to implement differential electrical drive with a series push-pull Mach-Zehnder optical modulator.
The prior art in this field also includes the following. An example of a GaAs/AIGaAs series push-pull travelling wave electrode Mach-Zehnder modulator was demonstrated by R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators”, IEEE J. Quant. Elect., vol. 27(3), pp. 654-667, 1991. In his FIG. 13, Walker shows an incoming electrical waveguide consisting of a single (non-differential) signal conductor with two ground conductors. One of the ground conductors is open-terminated without contact to the Mach-Zehnder modulator. It should be noted that the Walker solution cannot use both S+ and S− signals from a differential driver, thereby losing a large fraction of the electrical power, and suffers from poor modulation performance due to the open-terminated ground conductor.
S. Akiyama et al., “Wide-Wavelength-Band (30 nm) 10-Gb/s Operation of InP-Based Mach-Zehnder Modulator With Constant Driving Voltage of 2 Vpp”, IEEE Photon. Tech. Lett., vol. 17 (7), pp. 1408-1410, 2005, shows a Mach-Zehnder modulator similar to Klein, but does not show one of the conductors being grounded. However, the text of the paper describes “only one high-frequency signal” and does not teach how to transfer an electrical differential pair efficiently onto the Mach-Zehnder modulator.
I. Betty et al., “Zero Chirp 10 Gb/s MQW InP Mach-Zehnder Transmitter with Full-Band Tunability”, OFC/NFOEC 2007, paper OWH6. describes a Mach-Zehnder modulator for which each of the two arms can be driven by a separate signal, and then drives the two arms individually with each pair of a differential driver. However, in this case each arm receives the signal from a separate coplanar waveguide with its own ground conductors, and each arm is separately terminated by a 50 ohm resistor. Such a configuration is not compatible with a two-conductor series push-pull travelling wave electrode, and as a result suffers from a limitation on the bandwidth. Although figure la shows only two signal electrodes, there is also a grounded conducting substrate running beneath the signal electrodes, which negates the possibility of using a series push-pull Mach-Zehnder optical modulator and the high bandwidth thereof. In other words, this ground plane in the Betty device provides an electrical return path for each signal electrode, so the two signal paths do not act as return paths for each other.